1. Field of the Invention
The invention relates in general to an emission material and an organic electroluminescent device (OELD) using the same, and more particularly to a pure blue emission material and an organic electroluminescent device using the same.
2. Description of the Related Art
Conventional organic electroluminescent device (OELD) is a multi-layer stacked structure and includes a substrate, an anode, a cathode, a hole injection layer, a hole transport layer, an electron transport layer, an electron injection layer and an emission layer. The anode, the hole injection layer, the hole transport layer, the emission layer, the electron transport layer, the electron injection layer and the cathode are sequentially disposed on the substrate from bottom to up. The emission layer includes a host and dopant system, that is, the host is mixed with a small amount of dopant. As for how to determine whether the host and dopant system is a fluorescent host and dopant system or a phosphorescent host and dopant system is disclosed below.
When a voltage is applied to the cathode and the anode, the electron will pass through the electron injection layer and the electron transport layer to be injected into the emission layer from the cathode, and the hole will pass through the hole injection layer and the hole transport layer to be injected into the emission layer from the anode. After the electron and the hole are combined in the emission layer, the host will be excited to the exciton state from the ground state. Since the host is unstable at the exciton state, the host would return to the ground state from the exciton state and transfer energy to the dopant at the same time.
When the dopant receives the energy and is accordingly excited to the exciton state from the ground state, the dopant would generate both singlet excitons and triplet excitons. Regardless of the dopant being fluorescent or phosphorescent, the ratio of the probability of forming the triplet exciton to the probability of forming the singlet exciton is approximately 3:1 due to the distribution ratio of the electron spin state.
Both the singlet exciton and the triplet exciton return to the stable ground state by releasing photons, enabling the OELD to be electroluminescent. In the fluorescent host and dopant system, only the light emitted when the singlet exciton returns to the ground state is visible fluorescence. In the phosphorescent host and dopant system, the light emitted when the triplet exciton returns to the ground state is visible phosphorescence, so is the light emitted when the singlet exciton returns to the ground state visible phosphorescence after the conversion of internal system crossing (ISC).
Since the blue phosphorescent material has not achieved the standards of the fluorescent material in terms of color purity and lifespan, the red and green phosphorescent materials are mainly applied to the OELD according to their characteristics and lifespan have achieved commercializing standards.
The color of the conventional blue phosphorescent material iridium(III) bis(4,6-di-fluorophenyl)-pyridinato-N,C2′)picolinate (FIrpic) is not blue enough. As for other conventional blue phosphorescent materials, some materials are bluer when dissolved in a solution but are not blue enough when disposed in a device.